The invention relates generally to nondestructive methods for inspecting steel pipelines for plastically deformed regions caused by mechanical damage to the pipeline. More particularly, the invention is a method and system that uses nonlinear harmonic detection methods to detect mechanical damage in pipelines. The invention uses a time-varying magnetic field to sense magnetic properties of the pipeline. The odd-numbered harmonic frequencies are detected and their amplitudes are related to the magnetic condition of the material under test to determine mechanical damage.
Detection and characterization of plastically deformed regions caused by mechanical damage in pipelines, particularly gas transmission pipelines is important because of the danger to pipeline integrity posed by this type of defect. A plastically deformed region is a region in a metal that has been strained beyond the elastic limit and is now permanently deformed. The detection of this type of mechanical damage becomes even more critical because most of the reportable incidents of problems related to defects on operating pipelines are caused by third party contact with the pipeline that results in mechanical damage. Mechanical damage can be in the form of dents or gouges, or both.
Residual stresses and plastic deformation indicating mechanical damage in materials can be nondestructively measured by a variety of methods, including x-ray diffraction, ultrasonic and electromagnetic techniques. Because of the limited penetration depth of x-rays in metals, x-ray diffraction is restricted to measurements of surface stresses, generally in a laboratory environment. Ultrasonic techniques measure the velocity of ultrasonic waves in the metal and relate those measurements to stress. However, there are difficulties in differentiating stress effects from the texture of the material. With electromagnetic techniques, one or more of the magnetic properties (such as permeability, magnetostriction, hysteresis, coercive force, or magnetic domain wall motion during magnetization) of a ferromagnetic material are sensed and correlated to stress. These techniques rely on detecting the change in magnetic properties of the material caused by stress which is known as the magnetoelastic effect. Many of these techniques are difficult to implement without a controlled sample available while the measurement is being done. In addition, these techniques are less sensitive to changes in the magnetic properties of ferromagnetic materials caused by stress than nonlinear harmonics techniques. They are also not generally appropriate for rapidly survey stress states within pipelines, especially when the pipeline is being tested with a pigging device moving through the pipeline at a relatively high rate of speed.